Color, digital image display devices are well known and are based upon a variety of technologies such as cathode ray tubes, liquid crystal and solid-state light emitters such as Organic Light Emitting Diodes (OLEDs). In a common OLED color display device a pixel includes red, green, and blue colored OLEDs. By combining the illumination from each of these three OLEDs in an additive color system, a full-color display having a wide variety of colors can be achieved.
OLEDs may be used to generate color directly using organic materials that are doped to emit energy in desired portions of the electromagnetic spectrum. However, the known red and blue emissive materials are not particularly power efficient. In fact, broad bandwidth (white appearing) materials are known that have power efficiencies that are high enough by comparison to narrow bandwidth materials to produce a comparably power efficient OLED display by placing color filters over a broad bandwidth emissive material. Therefore, it is known in the art to produce OLED displays by building a display using an array of white-emitting OLEDs and placing color filters over the OLEDs to achieve red, green and blue light emitting elements in each pixel.
While power efficiency is always desirable, it is particularly desirable in portable applications because an inefficient display limits the time the device can be used before the power source is recharged. In fact, for certain applications the rate of power consumption may be more important than any other display characteristic with the exception of visibility. For this reason, under certain circumstances the end user may wish to reduce the power consumption of a display by making tradeoffs in display performance, which may have the effect of reducing the visibility or perceived quality of the display under various conditions.
Portable applications may require the display to be used in locations with high ambient illumination. It is known in the art, that an emissive display must be capable of providing higher luminance levels to be seen under high ambient illumination conditions than under lower ambient illumination conditions, and it is also known that these higher luminance levels are necessary to produce both adequate luminance contrast as well as a luminance range that is near the adapted luminance range of the observer. See “The ABC's of Automatic Brightness Control”, R. Merrifield and L. D. Silverstein, SID 88 Digest, 1988, pp. 178-180. For this reason, it is known to provide a user with a control to change the luminance of the display in response to changes in ambient illumination conditions. It is also known to automatically adjust the luminance of the display. For example, U.S. Pat. No. 3,813,686, issued May 28, 1974 to Mierzwinski, discusses a control circuit for a cathode ray tube that automatically increases the luminance and chrominance signals to produce a more appealing and useful image under high ambient viewing conditions.
In portable applications, such an automatic circuit allows the display to provide a lower luminance and thus reduced power consumption under low ambient illumination conditions and a higher luminance and thus improved visibility under high ambient illumination conditions. Many enhancements have been discussed for this basic method of adjusting the luminance of a display in response to changes in ambient illumination. For example, U.S. Pat. No. 6,411,306, issued Jun. 25, 2002 to Miller, et al., discusses a method of adjustment for a portable device in which the luminance and contrast of the display are modified in a way which is consistent with human adaptation, that is the luminance of the display is adjusted quickly and in a progressive fashion as the display is moved from a low to a high ambient illuminance environment but the luminance of the display device is adjusted more slowly as the display is moved from a high to a low ambient illuminance environment. However, any previous method that has been used to adjust the luminance of the display has required proportionally more power with increases in display luminance.
In a typical, prior-art OLED display, it is known that the luminance of the red, green, and blue OLEDs increase as current density delivered to the OLED is increased. The transfer function from current density to luminance typically behaves according to a linear function as shown in FIG. 1 as known in the prior art. FIG. 1 shows current density to luminance transfer functions for typical red 2, green 4 and blue 6 OLEDs. Therefore, to increase the luminance of the display, one must increase the current delivered to an OLED with a given area. To maintain a color-balanced display, the current must be adjusted differentially to the three OLEDs to maintain the desired ratio of red:green:blue luminance.
Unfortunately, increasing the current density used to drive an OLED not only increases the power required to drive the OLED but also reduces the lifetime of the OLED. FIG. 2 shows typical functions that describe the time required for an OLED to lose half of its luminance as a function of the current density used to drive the OLED. These functions describe the luminance stability over time of the OLEDs as a function of current density. FIG. 2 shows the luminance stability over time of a typical red 8, green 10 and blue 12 OLED. Therefore, increasing the luminance of an OLED display not only increases the power needed to drive the OLED display device but can significantly reduce the lifetime of the OLED display device.
The overall lifetime of a display may be decreased through changes in relative color efficiency as well as declining luminance output. If one OLED material used to produce a particular color of light degrades more rapidly than other materials that produce other colors of light, for example through heavier use, the particular light output from the material will decrease relative to the other colors of light. This differential color output change will change the color balance of the display and is much more noticeable than a decrease in overall luminance. While this decrease in luminance and light output of the particular color can be accommodated by increasing the brightness of the particular color, such a solution increases the rate of aging, the power usage, and exacerbates the change in relative color efficiency in the display.
There is, therefore, a need to improve power efficiency and/or improve display lifetime of an OLED. There is a further need for a full-color OLED display device having improved power efficiency and lifetime without necessarily decreasing the luminance of the display.